Energy harvesting is a technique for supplying distributed embedded microsystems with energy by means of energy conversion at their respective place of use. For this purpose, electrical energy is obtained from another form of energy present at the place of use, for example, from thermal, optical, chemical or mechanical energy into electrical energy by means of a respective generator.
A plurality of generator types exists in order to convert the above-mentioned energy forms into electrical energy. For the present invention, for instance, thermoelectric and photovoltaic converters are relevant as well as electrochemical cells, such as so-called biofuel cells. In most cases, these generators output DC voltage. Only in the case of a thermoelectric converter, the polarity of the output voltage is inverted, when the direction of the temperature field is inverted, which is present across the generator. All above generated principles have in common that one single converter—depending on the design and the nature of the supplied known electric input power—can yield output voltages, which are significantly below the level that is needed for operating low voltage CMOS electronics of an embedded microsystem. Furthermore, the output voltage of various generators depends on the level of the input energy. Varying energy input therefore leads to a varying output voltage of the generator.
For the above-mentioned generators, as well as in other cases, it is necessary to increase with a circuit for voltage conversion, the low output voltage of a source to such a point that an electronic circuit can be supplied with a sufficiently high voltage. A schematic representation of the basic arrangement is shown in FIG. 1. In particular, an electronic voltage converter is arranged between the generator and the electronic circuitry, which in the following is referred to as the load resistor RL. The output of the generator is connected to the input of the voltage converter. The output of the voltage converter is connected to the load. As a result, the variable input voltage Vin, which is provided to the generator, is applied at the input of the voltage converter. Within the voltage converter, Vin is transformed into a higher output voltage Vout that is applied to the load RL. The electronic system at the output of the voltage converter can additionally contain an electrical energy storage device, e.g. a rechargeable battery or an electrical capacitor. In this case, the voltage converter feeds the energy storage device and the load via its output. If the input energy at the generator breaks down, energy from the energy storage device is available in order to ensure the continuous operation of the voltage converter by feeding from the output or via a separate feed entry. This would likewise ensure that the converter circuit is functional again immediately and starts up when there is again sufficient input energy available from the generator. However, if this temporary storage is not available or has been discharged excessively, then it is necessary for the voltage converter to draw its operating energy completely from its input and already take on the function at the lowest possible input voltages.
From today's state of the art, there are known various circuit concepts which allow transforming low input voltages into higher output voltages.
One concept that is frequently used is the so-called inductive step-up converter, also known as boost converter, which is available as an integrated circuit in numerous embodiments. A description can be found in. A description can be found in U. Tietze, Ch. Schenk, “Halbleiter-Schaltungstechnik”, Springer-Verlag, 11th Edition, 1999, page 985 and onwards. The basic circuit, which is reproduced in FIG. 2 comprises a switching transistor in bipolar or MOS technology, an inductor, a diode and a capacitor. A control circuit ST for generating square wave signals Vcontrol is furthermore required, which is supplied from an operating voltage VB.
Without going into further detail about this step-up converter, it is noted that the control circuit ST needs a supply voltage VB with a sufficiently high amplitude for generating the square wave signals Vcontrol. This is a significant problem for step-converters that have to be supplied exclusively from the input voltage Vin. The starting voltage, i.e. the lowest necessary input voltage, is determined decisively by the required operating voltage of the control circuit and the necessary amplitude of the control voltage Vcontrol and cannot be reduced arbitrarily. In different circuitry concepts, auxiliary circuits are used for supporting the startup phase at low voltages. However, for such an exemplary circuit, the integrated circuit TPS 61200 of Texas instruments, the lowest input voltage Vin, which is needed, still amounts to about 0.3 volts without a load at the output Voutput, and to 0.5 volts with a load at the output.
In a step-up converter, the magnetic field in the core of the coil always oscillates around a mean value, which correlates to the mean value of the coil current. Consequently, the coil core always stays magnetically biased in one direction. The core of the coil therefore has to be designed in a way that even when the magnetic field oscillates around a mean value, no lossy magnetic exaggeration of the core occurs. This leads, for instance, to a correspondingly larger size of the core.
In the article IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, vol. 33, no. 5, SEPTEMBER/OCTOBER 1997, a resonant switched-mode converter principle based on a modified Meissner oscillator is presented, which has been developed in particular for operating at low starting voltages. The corresponding electronic component is called “starter circuit” and is shown in FIG. 3. In particular, the drain-source path of an n-channel junction field-effect transistor T1 (n-JFET) is connected in series to winding 1 of a transformer Tr and subjected to electrical voltage via the input Vin of the converter circuit. A winding 2 of the transformer Tr with a substantially higher number of windings than the winding one is interconnected to the gate of the n-JFET T1 as feedback. This is achieved with a winding in a sense opposite to that of the primary winding. As a result, the positive voltage at winding generates a negative voltage at winding 2 and vice versa. The reference point of winding 2 is connected to the reference earth of the circuit via a parallel circuit with a capacitor C3 and a resistor R1, while the high point is connected to the gate of the n-JFET T1.
For this circuit in the above-cited article, a starting voltage Vin of about 300 mV is mentioned. The circuit takes advantage of the fact that n-JFET is already conducting at a gate-source voltage of 0 V. Thus, a current flow through winding 1 of the transformer Tr and through the n-JFET T1 is already starting at low input voltages and therefore a positive voltage is generated at winding 1. The magnetic field, which is building up induces in the feedback winding 2 of the transformer a negative voltage, which is larger than the voltage at the primary winding 1 depending on the ratio of the turns of both windings. The gate source path of the n-JFET T1 represents a pn diode, wherein the anode is connected to the gate. This diode limits the voltage VGS at the gate of T1 to about +0.6 V against ground. The higher transformed voltage at winding 2 thereby charges the capacitor of the RC element (C3 and R1) to negative voltages VRC against ground. As soon as the current flow through winding 1 reaches an equilibrium, the voltage induced in winding 2 breaks down. Thus, the negative potential VRC, which has been built up at the capacitor C3, energizes through the gate n-JFET T1 and polarizes the pn transition in the reverse direction. The closer this negative gate voltage comes to the negative terminal voltage of the n-JFET, the more transistor T1 is blocked. The resulting decrease of the current in winding 1 induces a positive voltage in winding 2. This positive voltage at winding 2 is added in reverse polarity to the already existing negative gate bias. As a consequence, VGS further changes in the direction of negative values until transistor T1 is blocked abruptly at a certain point in time. The RC element formed by C3 and R1, now discharges at its RC time constant, as a result of which the gate source voltage VGS at transistor 1 is changed from negative values back towards 0 volt with its time delay. As a consequence, the current flow through winding 1 gradually increases again as T1 becomes conductive. The described process repeats.
In a winding 3 of the transformer, the self-controlled oscillation induces a further alternating voltage, with which, due to the higher winding ratio, lies above the input voltage at winding 1 by an adjustable factor. This voltage is rectified with a diode D and used as a stepped-up output voltage. The capacitors C1 and C2 buffer the voltages Vin and Vout, respectively. Similar concepts based on Meissner oscillators are presented in other publications, such as “STEP-UP DC-DC-CONVERTER WITH COUPLED INDUCTOR FOR LOW INPUT VOLTAGES,” Proceedings of PowerMEMS 2008+microEMS 2008, Sendai, Japan, Nov. 9-12, 2008, pp. 145-148, in the article “DC-DC-CONVERTER WITH INPUT POLARITY DETECTOR FOR THERMOGENERATORS,” Proceedings PowerMEMS 2009, Washington D.C., USA, Dec. 1-4, 2009, pp. 419-422 or in the article “ULTRA-LOW INPUT VOLTAGE DC-DC CONVERTER FOR MICRO ENERGY HARVESTING,” Proceedings PowerMEMS 2009, Washington D.C., USA, Dec. 1-4, 2009, pp. 265-268. Furthermore, to commercially use ICs of the company Linear Technology having the type designation LTC 3108 and LTC 3109 use a Meissner oscillator with a modified configuration. For the IC LTC 3108, for instance, a value of 20 mV is given as startup voltage.
However, these concepts are mostly realized as self-starting Meissner oscillators in a non-linear large signal operation. This means that the current flow through the input winding of the transformer alternatingly starts and breaks down again. Consequently, the transformer is not supplied with alternating current continuously, but is operated in a DC mode with superimposed alternating component. From this, a pre-magnetization of the core of the transformer results having all known disadvantages regarding the efficiency of the converter circuit and the dimensioning of the transformer.
For operating a transformer in a pure alternating voltage mode, different concepts of forward converters are known. These either use a transformer with two input windings, which are supplied with current alternatingly so that in the core of the transformer, a magnetic alternating field is induced, or they use a transformer with only one input winding, which is supplied with alternating current via an H bridge. This configuration therefore avoids the disadvantage of a pre-magnetization of the magnetic material within the transformer core. The control signals for the transistors at the input windings are generated with separate control circuits.
FIG. 4 shows a corresponding basic circuit of a single-ended forward converter according to the state of the art, as described for instance in U. Tietze, Ch. Schenck, “Halbleiter-Schultungstechnik,”11th edition, 1999, page 990.
In this circuit, a transformer is operated with three windings. Winding 3 provides in the shown example the output voltage Vout via a full way rectifier with four diodes. Winding 1 is alternatingly connected to the input voltage Vin and is disconnected therefrom again via transistor T1. Winding 2 is connected between the input voltage Vin and ground via a diode D. In winding 2 as well as in winding 3, an induced alternating voltage is generated. This alternating voltage is short-circuited whenever a negative voltage is induced at the cathode of diode D. By suitably selecting the winding directions of windings 1 and 2, this is always the case if the transistor T1 blocks. The corresponding current flow through winding 2 and diode D causes the magnetic field within the coil core to invert its plurality because the demagnetization current in winding 2 acts in the opposite direction compared to the current in winding 1. Further, via the current flowing in winding 2, energy is fed back into the input voltage Vin. On average and ideally, the resulting magnetization of the core is 0, having the advantage that the core of the transformer can be reduced in size and that the danger of a saturation of the core can be avoided.
Further concepts are shown in the publication U. Schlienz, “Schaltnetzteile und ihre Peripherie,” 3rd edition, Vieweg-Verlag, 2007, page 91 and page 96. In these concepts, which are shown in FIGS. 5A and 5B, a transformer with two windings is operated in a push-pull mode. In a concept according to FIG. 5A, this is achieved by connecting a terminal of the input winding of the transformer with a capacitive voltage divider C1/C2 between the input voltage and ground, whereas the other terminal is alternatingly connected between the input voltage and ground via a half-bridge comprising MOSFET transistors T1 and T2. In a further concept, as shown in FIG. 5B, the input voltage is connected with changing polarity to the input winding of the transformer via a full-bridge. For rectifying the output voltage of the transformer, a plurality of concepts is known.
These forward converters also require a control circuit ST generating the corresponding square wave signals Vcontrol and applying same to the gate terminals of the used MOSFET transistors. Hence, this circuit concept has the same problems as the above-described step-up converter. When the whole circuit is to be operated from the input voltage Vin, the required operating voltage VB of the control circuit defines the lowest possible starting voltage.
In summary, when contemplating all known voltage converters, it can be seen that same require for controlling the internal processes within the circuits a controlled voltage that is above a defined minimal value. When using bipolar transistors and even with MOSFETs having a low control voltage, this required control voltage typical control voltage amounts to 0.3 V up to 0.6 V. This voltage is usually generated by means of control circuits and derived from an available separate operating voltage. A self-starting of these circuit concepts from input voltages in the region of several 10 mV is therefore not possible with most of the concepts. The only exceptions are the above-described resonant converters according to the principle of a Meissner oscillator. Additionally, within a controlled circuit, a continuous internal power loss occurs, which is disadvantageous regarding the efficiency of the voltage converter. Accordingly, the lowest possible starting voltage of almost all integrated or discretely mounted low voltage step-up converters is today about 0.6 V. By means of an additional auxiliary circuitry, the minimal starting voltages of about 0.3 V can be achieved. Lower starting voltages cannot be achieved according to the present state of the art. Forward converters with such low starting voltages do not exist at all up to now.
A further disadvantage of known concepts can be seen in the fact that most of the mentioned circuit concepts require transformers with more than two windings or with split windings. This increases the price and the size of the transformer. In the prior art, miniaturized transformers are known, which have only two windings and were developed particularly for converters with low starting voltages. However, for realizing a small size, they are limited regarding the ratio of the windings and the number of turns. The circuit concepts shown in the data sheets of the company Linear Technology with a LTC 3108 and LTC 3109, for instance, use a particular transformer of the company Coilcraft with maximum winding ratios of 1 to 100 between an input winding and an output winding.
A third advantage can be seen in the fact that the cited self-starting Meissner oscillators allow a low staring voltage. However, at the same time, are operated in the non-linear large signal operation. This means that the current flow through the input winding of the transformer alternatingly starts and breaks off again. Thus, the transformer is not supplied continuously with alternating current, hut is operated in a DC mode with superimposed alternating components. As a result, a pre-magnetization of the core of the transformer occurs having all known disadvantages regarding the efficiency of the converter circuit.
Consequently, an object underlying the present invention is to provide a voltage converter circuit that overcomes the above problems and disadvantages and allows in particular for energy-harvesting applications an efficient and economic voltage conversion with a self-start even at low input voltages.
This object is solved by the subject matter of the independent claims. Advantageous embodiments of the voltage converter according to the present invention are the subject matter of the dependent claims.